A ball rolls off the top of a stairway with horizontal velocity \( u \). The steps are \( 0.1 \, \text{m} \) high and \( 0.1 \, \text{m} \) wide. The minimum velocity \( u \) with which that ball just hits the step \( 5 \) of the stairway will be \( \sqrt{x} \, \text{m s}^{-1} \) where \( x = \) ________________ [use \( g = 10 \, \text{m/ s}^{2} \) ].
